A combination of things-
-rinsing once or twice a day
-lots of elbow grease with a rice root brush.....brush, brush, brush and then do it some more
-keeping the animal in a dark area during the daylight
-if you can't put them in a cooler, then fans in a dark area will work also.
-good genetics in a calf.....some calves are simply hairier and some just don't have it. We have found really hairy ears when they are little to be a decent indicator of genetics with good hair.
Don't think it will come without a lot of work and a combination of all of the above. There is no magic potion.....elbow grease with a brush is your best bet.